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Scientists studying nearly 250,000 people in France found that only overweight people who also had high blood pressure were at significantly greater risk of dying of heart-related problems than normal-weight people. Overweight people with normal blood pressure faced no increased risk.

This doesn’t mean that extra pounds aren’t dangerous, because overweight people are more likely to develop blood pressure problems.

But it does for the first time show that blood pressure may be an important “mediator” or mechanism by which excess weight can cause heart problems, said one expert who reviewed the work, Dr. Frank Hu, an associate professor at the Harvard School of Public Health.

“This novel finding deserves careful consideration,” he wrote in an editorial accompanying the findings of the study, published Tuesday in Hypertension: Journal of the American Heart Association.

Health experts have long agreed that obesity raises the risk of dying, but they argue about how dangerous it is to be merely overweight. A controversial study earlier this year by the Centers for Disease Control and Prevention concluded that people who were overweight but not obese might even be less likely to die than those who are thinner.

The new study found that overweight people have a greater risk of dying in general as well as from cardiovascular causes. But when researchers took into account things that raise heart risks, like high cholesterol, diabetes and high blood pressure (hypertension), only high blood pressure made a difference in the risk of dying.

“This study shows that hypertension is most important and is extremely common among people who are overweight and obese,” Hu said.

The new study was led by Dr. Frederique Thomas at the Medical School of Nancy and involved 139,562 men and 104,236 women who had routine health checkups at a clinic in Paris from 1972 to 1988. The average age for men was 43 and for women, 41.

Forty-two percent of the men and 21 percent of women were overweight, but the study included relatively few people who were obese, so researchers made no conclusions about that group.

During an average of 14 years of follow-up, 2,949 men and 929 women died from cardiovascular disease.

Overweight people with high blood pressure had twice the risk of dying of a heart attack or stroke than overweight people with normal blood pressure. Those who had high blood pressure plus other problems like diabetes had even greater risk.

In the study, half of overweight people had high blood pressure, and Hu said many more likely would develop it in later years. Extra pounds do this in a number of ways — by raising insulin production, causing kidney problems and increasing salt sensitivity, to name a few.

That not everyone overweight develops high blood pressure also may help explain why not everyone who is overweight suffers health problems, Hu said.

“It’s like saying not all the smokers will develop lung cancer,” even though smoking clearly raises that risk, he said.